Users should be less concerned if the application they're using is from a Linux distributor, because they have patches available. But with third-party vendors, users might not know about the problem until they read about it.
Vendors can take months to create patches, and sometimes users grumble about that, ... But the alternative is to have patches that can be circumvented or aren't appropriate for the vulnerability. It's a difficult balance.
Why bother writing a virus for Linux and Mac when you can get so many other users by writing one for Windows? This is especially true because, for a virus to become serious, it has to find other vulnerable systems, and with Macs, that would be a very limited spread.
This is big problem because a very large number of corporations use Lotus Notes. When users receive an e-mail with an attachment, all they have to do is click on the attachment to read it, and their systems are vulnerable to a remote attack.